While using your Kindle it be become Frozen and unresposive. You have try to press all the different buttons but still it is not doing anything. Here is what you can try to see if you could get it working again:
1. If the Kindle is connected to a PC, unplug it.
2. Next push the Power button the the right and let go. Now the green LED will show up for around two seconds.
3. Next push the Power button to the right again and hold it this time for around 5 seconds until you see the green light blinks 3 times. Let go of the Power button when the screen goes blank.
4. Now switch the Kindle back on again, by simply push the Power button to the right and let go.
If you still have a problem with device then you may want to consider to reset it. Kindle: How To Hard Reset Kindle.